One of the problems encountered with some chip carrier substrate interconnections to the next level of packaging is the high stress on the interconnections caused by coefficient of thermal expansion (CTE) mismatch between the substrate and the next level of packaging. The CTE thermal mismatch is particularly large where a ceramic chip carrier substrate is connected to a printed wiring board typically made of an epoxy/glass material. When a high circuit density chip is attached to the chip carrier substrate, the heat generated by the chip compounds the CTE mismatch problem between the chip carrier substrate and the printed wiring board because of large temperature variations between the chip and the printed wiring board. In addition, certain applications, such as flip chip applications, have required encapsulation to ensure a reliable flip chip interconnection in the solder joints between the chip and the substrate. Such encapsulation typically employs a high strength epoxy which acts to reinforce the bond between the chip and the chip carrier substrate. During thermal cycling, this reinforcing of the chip to chip carrier substrate reduces solder joint stress between the chip and the chip carrier. When coupled with the CTE mismatch between a ceramic chip carrier attached by a solder interconnection to a printed wiring board, this reinforcement can cause high stress between the ceramic chip carrier substrate and the printed wiring board to which it is attached. Repeated thermal cycling can lead to cracking of the solder interconnection between the ceramic chip carrier and the printed wiring board, ultimately affecting reliability of the chip/substrate/printed wiring board package.
The above described high stresses on the solder interconnections are generally attributed to the fact that the bonding of chip to the ceramic chip carrier, including the encapsulant, forms a composite which acts during thermal cycling to cause this composite to act like a bimetallic element wherein the composite bends due to the different CTE of the materials. As a result of the large thermal mismatch between the composite and the printed wiring board, the thermal cycling induced bending over time can cause solder interconnection failure. In this regard, the CTE for a typical chip may be in the order of about 3 parts per million per degree Centigrade (ppm/degree C.). The CTE for a typical ceramic chip carrier may be in the order of about 3-5 ppm/degree C., while a typical printed wiring board CTE may be about 18-22 ppm/degree C.
In general, others have attempted to address the problems caused by CTE mismatch of materials in electronic packages by providing various interposing structures that attempt to reduce the CTE mismatch. For example, multiple layers of materials with varying CTEs may be employed to form an interposing layer between one level of packaging and the next, with the layers having a gradation of CTEs such that the layer contacting one level of packaging is selected to have a CTE which more closely matches the CTE of that level while the layer contacting the next level of packaging has a CTE more closely matching that level while layers between may gradually reduce the difference. In addition, efforts have also been made to use interposing layers which are flexible in nature such as to reduce the stress on electrical interconnections during thermal cycling created by thermal mismatch. However, these various efforts typically are either difficult to assemble or are not totally effective in their purpose.
The present invention is directed at overcoming the problems set forth above. It is desirable to have an electronic package and method to make the electronic package that significantly absorbs the stresses that occur between a chip carrier and a printed wiring board during thermal cycling. Electronic packages produced by this method will have increased operational field life.